The present invention relates to a velocity interferometer with continuously variable sensitivity. It is applicable to the study, without mechanical contact, of the movement of an optically reflecting surface or a rough surface able to back scatter an incident light beam. It makes it possible to carry out measurements, even when the state of the investigated surface evolves under a mechanical action (stress, shock) or chemical action (phase change, oxidation).
Velocity interferometers are known called "wide angle MICHELSON interferometers" or "widened field interferometers", which make it possible to perform such measurements. One exemplified construction of such interferometers is diagrammatically shown in FIG. 1. It essentially comprises a laser 2 transmitting, via an e.g. semi-transparent mirror 3, a light beam onto a diffusing surface 4 to be investigated, which can e.g. be moved with the aid of a projectile 5. A beam splitting means 6, which traps the light reflected by the surface illuminated by the light beam, reflects one part thereof and transmits the other part. A first mirror 7 reflects in the direction of the splitting means 6 the light reflected by the same and then transmits the other part. A second mirror 8 reflects in the direction of the splitting means 6, the light transmitted by the same and reflects the other part. There are means 9 for the detection of the interferences resulting from the superimposing of the light from the splitting means 6 after reflection on the first mirror 7 and the light from the splitting means 6 after reflection on the second mirror 8. There is a glass block 10 with parallel faces of length or more accurately thickness L (distance between two parallel faces) and optical index n, interposed between the splitting means 6 and the second mirror 8, the distance from the first mirror 7 to the splitting means 6 being equal to the distance between the latter and the apparent position 11, viewed from detection means 9, of the second mirror 8.
Such an interferometer suffers from the following disadvantages. In order to have a high sensitivity, it is necessary to have a glass block 10 of considerable length (e.g. approximately 1.50 m for detecting velocity variations .DELTA.V less than 30 m/s), which is difficult and costly. In addition, the sensitivity of the interferometer is fixed once and for all by the choice of a glass block, bearing in mind the following relation: EQU .DELTA.V=1/4.lambda..sub.0 c L.sup.-1 (n-1/n).sup.-1 ( 1)
in which .lambda..sub.0 and c respectively designate the emission wavelength of laser 2 and the velocity of light in vacuo.